Discussion

DC
An area of heavy showers and strong thunderstorms is now closing in on the DC metro region from west-to-east. This band of showers and thunderstorms is associated with a strong cold frontal system and it’ll arrive in the western suburbs of DC by 11am or so and then spread across the rest of the DC metro region during the mid-day hours. This line has prompted the National Weather Service (NWS) to put up a “tornado watch” for entire the DC metro region until 5PM with the greatest threat for severe weather from I-95 to points east.

Philly
Farther north, the showers and thunderstorms will arrive in the Philly metro region during the early-to-mid afternoon hours (~1-3PM). Brief downpours are likely and there can be a strong thunderstorm or two mixed in. There is still some low-level cold air around in the Philly metro region as compared with DC and this should reduce the chance for severe weather north of the Mason Dixon line (11am observations: 40 degrees at Philly Airport, 63 degrees at Reagan National in DC), but it is possible that the NWS extends the "tornado watch" into SE PA later today.